Of the efforts currently being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. Deep-UV lithography is capable of fabrication to dimensions of 0.2 μm or less and, when a resist having low light absorption is used, can form patterns with sidewalls that are nearly perpendicular to the substrate. One technology that has attracted a good deal of attention recently utilizes high-intensity KrF and ArF excimer lasers as the deep-UV light source. This technology is being used in large-volume production, prompting a desire for resists having a low light absorption and a high sensitivity.
Acid-catalyzed, positive-working chemical amplification resists (e.g., JP-B 2-27660 and JP-A 63-27829) developed in response to the above needs are endowed with excellent properties, including a high sensitivity, high resolution and good dry-etching resistance, which make them especially promising as resists for deep-UV lithography.
However, one problem with chemical amplification resists is that, when the standing time from exposure to post exposure bake (PEB) is long, the line pattern formed during patterning acquires a “T-top” shape characterized by widening at the top of the pattern. This defect is called “post exposure delay” (PED). Another problem with such resists is “footing,” which is a widening of the resist pattern close to the substrate that occurs on a basic substrate, particularly a silicon nitride or titanium nitride substrate. The T-top effect is believed to result from a decrease in solubility at the surface of the resist film, and the footing effect at the substrate surface appears to arise from a decline in solubility near the substrate. An additional problem is that acid-labile group elimination is a dark reaction which proceeds during the interval between the exposure step and the PEB step, reducing the final dimensions of the pattern lines.
These problems represent major drawbacks to the practical use of chemical amplification resists. Because of such defects, prior-art positive-working chemical amplification resists are difficult to dimensionally control in the lithographic process, and dimensional control is also lost during dry etching of the substrate (see, for example, W. Hinsberg et al., Journal of Photopolymer Science and Technology, Vol. 6, No. 4, 535–546 (1993); and T. Kumada et al., ibid., 571–574).
In positive-working chemical amplification resists, the problems of PED and footing on the substrate surface are believed to be caused in large part by basic compounds which are either airborne or present on the surface of the substrate. The acid at the surface of the resist film that has been generated by exposure reacts with airborne bases and is deactivated. Prolonged standing until post-exposure bake results in a corresponding increase in the amount of deactivated acid, making it more difficult for the acid-labile groups to decompose. A substantially insolubilized layer thus forms at the surface, giving the resist pattern a T-top shape.
It is well-known in the art that the addition of a basic compound can check the influence of airborne bases, and is thus effective also against PED (see, for example, JP-A 5-232706, JP-A 5-249683, JP-A 5-158239, JP-A 5-249662, JP-A 5-257282, JP-A 5-289322, JP-A 5-289340, JP-A 6-194834, JP-A 6-242605, JP-A 6-242606, JP-A 6-263716, JP-A 6-263717, JP-A 6-266100, JP-A 6-266111, JP-A 7-128859, JP-A 7-92678, JP-A 7-92680, JP-A 7-92681, JP-A 7-120929 and JP-A 7-134419).
Familiar basic compounds include nitrogenous compounds such as amine compounds and amide compounds. Specific examples include pyridine, polyvinylpyridine, aniline, N-methylaniline, N,N-dimethylaniline, o-toluidine, m-toluidine, p-toluidine, 2,4-lutidine, quinoline, isoquinoline, formamide, N-methylformamide, N,N-dimethylformamide, acetamide, N-methylacetamide, N,N-dimethylacetamide, 2-pyrrolidone, N-methylpyrrolidone, imidazole, α-picoline, β-picoline, γ-picoline, o-aminobenzoic acid, m-aminobenzoic acid, p-aminobenzoic acid, 1,2-phenylenediamine, 1,3-phenylenediamine, 1,4-phenylenediamine, 2-quinolinecarboxylic acid, 2-amino-4-nitrophenol, and triazine compounds such as 2-(p-chlorophenyl)-4,6-trichloromethyl-S-triazine.
Of these, pyrrolidone, N-methylpyrrolidone, o-aminobenzoic acid, m-aminobenzoic acid, p-aminobenzoic acid and 1,2-phenylenediamine are preferred.
These nitrogenous compounds are weak bases and can alleviate the T-top problem, but such compounds are unable to control the reaction when highly reactive acid-labile groups are used; that is, they cannot control acid diffusion. With the addition of a weak base, the dark reactions in PED in particular proceed in unexposed areas, causing slimming of the line dimensions and a loss of film thickness from the line surface during PED. To overcome such problems, it is desirable to add a strong base.
However, a higher basicity is not necessarily better. For example, good effects cannot be obtained with the addition of the following super-strong bases: DBU (1,8-diazabicyclo[5.4.0]-7-undecene), DBN (1,5-diazabicyclo[4.3.0]-5-nonene), 1,8-bis(dimethylamino)naphthalene or quaternary ammonium hydroxides such as tetramethylammonium hydroxide.
The addition of a base having an excellent generated acid capturing effect works well to increase the contrast and thereby achieve a high resolution. The dissociation constants of the acid and base within water can be explained in terms of pKa, but the acid capturing ability within the resist film is not directly related to the pKa of the base. This is discussed by Hatakeyama et al. in Journal of Photopolymer Science and Technology, Vol. 13, No. 4, pp. 519–524 (2000).